Quiet periods in edge turbulence preceding the L-H transition in the National Spherical Torus Experiment

Abstract

This paper describes the first observations in the National Spherical Torus Experiment (NSTX) [S. M. Kaye et al., Phys. Plasmas 8, 1977 (2001)] of “quiet periods” in the edge turbulence preceding the low-to-high (L-H) mode transition, as diagnosed by the gas puff imaging (GPI) diagnostic near the outer midplane separatrix. During these quiet periods the GPI Dα light emission pattern was transiently similar to that seen during H-mode, i.e., with a relatively small fraction of the GPI light emission located outside the separatrix. These quiet periods had a frequency of ∼3 kHz for at least 30 ms before the L-H transition, and were correlated with changes in the direction of the local poloidal velocity. The GPI turbulence images were also analyzed to obtain an estimate for the dimensionless poloidal shearing S=(dVp/dr)(Lr/Lp)τ. The values of S were strongly modulated by the quiet periods but did not significantly vary during the ∼30 ms preceding the L-H transition. Since neither the quiet periods nor the shear flow increased immediately preceding the L-H transition, neither of these appears to be the trigger for this transition, at least for these cases in NSTX.